Fixes #3411 .  Corrected uo4.i

Put some error-checking stuff in RichardsMaterial so this won't happen again
and also some LIBMESH_DIM instead of "3".

Also slightly altered RichardsSeffPrimPrimeAux.C to avoid a comparison
between signed and unsigned, which was not the cause of this problem, btw.
Merge pull request #3419 from WilkAndy/uo4_crash_3411

Fixes #3411 .  Corrected uo4.i, also slightly altered RichardsSeffPrimPr...